The present invention relates to demodulation of digital signals. Furthermore, the invention relates to demodulation of digital signals modulated by a modulation method such as quadrature amplitude modulation (QAM) and transmitted with a plurality of carrier waves by orthogonal frequency-division multiplexing (OFDM).
The OFDM transmission is a frequency-division multiplexing transmitting method to transmit modulated digital data with a plurality of orthogonal carrier waves.
The OFDM transmission is advantageous in that it is useful in multipath transmission, it rarely has an effect on other transmission lines and vise versa, and it provides high frequency usage efficiency. It is thus useful for mobile digital audio and TV broadcasting.
The orthogonal carrier waves are generated by an inverse fast Fourier transform (IFFT) circuit in signal transmission. The carrier waves are separated from each other by a fast Fourier transform (FFT) circuit with Fourier transform in signal reception.
The OFDM transmission is useful for multipath transmission because of a guard interval that is a signal period of time provided per signal to be transmitted to cancel signal interference.
However, QAM requires waveform equalization (signal adjusting) in demodulation. Further, received signal level will tremendously vary due to movement in case of mobile signal reception under multipath environment. Thus, signal compensation more accurate than that in static condition is required.
The higher the travel speed during mobile communications and the greater the difference in received electric field strength under multipath environment, the greater the received signal variation per unit of time. Further, the more multilevel in QAM, the more difficult the modulation becomes.
In order to deal with the signal variation, a reference signal is inserted into a signal to be transmitted per carrier during signal transmission. Then, on signal reception, its amplitude phase variation is extracted to adjust signals coming later.
However, the reference signals are transmitted at a constant period of time. And, hence adjusting values will not be updated until the next reference signal comes.
The received signals that vary in a minute period of time thus cannot be demodulated due to movement during mobile communications or change in environment.
The more reference signals, the higher the possibility of adjusting, however, the less the transmissible data amount. Therefore, in mobile signal reception under OFDM, the more the multilevel becomes in modulation, the more difficult to absorb received signal variation.